Stars and interstellar matter

The Sun is one of about 200 billion stars in the Milky
Way galaxy. Between the stars are vast clouds of interstellar dust and
gas -- the material out of which new stars are made. The conversion of
interstellar matter into stars is one of the most fundamental topics in
modern astronomy, not only for what it tells us about the birth of the
Solar System, but because so many of the large-scale properties of galaxies
are the direct result of the star-formation process

Star Formation

The
earliest stages of star formation take place deep within molecular clouds,
from which little or no light emerges. Infrared radiation passes much
more freely through interstellar clouds than does light, so several
astronomers
at UH, including Klaus
Hodapp, John
Rayner and Alan
Tokunaga use infrared telescopes to study the star formation
processes. Broadly, the goals of these programs are to understand
the
sequence of events that leads from an interstellar cloud via a protostar
to a mature star. The evolution of a single object through these
stages
takes millions of years, so astronomers need to collect data from different
objects at different ages and try to place them in the correct sequence.
Some - perhaps all - newly formed stars go through a turbulent adolescence
in which they eject streams of gas that collide with the surrounding
molecular
cloud material. Klaus Hodapp uses infrared cameras to detect the 2.2
micron emission from hydrogen molecules that are excited by these
collisions,
while Alan Tokunaga uses infrared cameras and spectrographs to study
the faintest members of dark clouds. Some of these objects may be
substellar--
brown dwarfs or massive planets.

The
dust and molecules in these star forming clouds produce strong
emission at sub-millimeter wavelengths. The high altitude of Mauna
Kea makes observations at these frequencies feasible despite significant
atmospheric absorption and the mountain hosts three sub-millimeter telescopes
(CSO, JCMT,
and SMA) including the only
interferometer in the world to operate at these wavelengths. Jonathan
Williams uses
these telescopes to study molecular
clouds so as to learn about how they form stars, and also to observe
disks
around young stars in order to learn about the processes of planet
formation.

After a few million years, new stars start to emerge from
the molecular clouds in which they were born. George
Herbig is engaged in studies of very faint very young stars in
several Galactic clusters that are still partially embedded in the dense
molecular clouds from which they formed a few million years ago. These
stars can be detected spectroscopically from their characteristic signature
of a bright H-alpha spectral line, which is believed to be produced either
in an expanding wind or a deep chromosphere. These new detections reveal,
among other things, the relative numbers of stars of different masses
formed in a molecular cloud.

Bo
Reipurth leads the Center for Star and Planet Formation at the Institute for Astronomy, a group of faculty, postdocs and students
who share a common interest in the origins of stars and planets. He
studies the highly collimated Herbig-Haro jets that emerge from
newborn stars. These HH jets plow through the ambient medium, thus
helping to excavate cavities around the young stars and make them
emerge from their placental gas and dust clouds. HH flows can attain
gigantic proportions, stretching over tens of lightyears. They consist
of luminous shocks, each of which represent an explosive event in the
newborn star. HH jets can thus be read as a fossil record of activity
of young stars.

Young star clusters

Young
galactic clusters and stellar associations provide ideal locations
for star formation research because they
contain large numbers of coeval stars
evolving under similar conditions and with identical chemical compositions.
By using young clusters of different ages as snapshots in time, George
Herbig and graduate student Scott Dahm are
able to study large populations of solar-like stars as they
emerge from
their parental
molecular clouds as T Tauri stars and ultimately evolve into Zero-Age
Main
Sequence stars (ZAMS). These young, low-mass stars often exhibit
hydrogen line
emission and enhanced X-ray luminosities resulting from accretion
processes and
chromospheric activity. Excess infrared emission suggests the presence
of
circumstellar dust and gas which is thought to dissipate over the
first ten
million years of a star's lifetime. During this critical period,
it is believed
that planet formation occurs in solar-like stars. By studying these
young
stellar populations we are given insight into the early history
of the Sun
and our own solar system.

Michael Liu focuses on understanding the nature
and origin of substellar objects, i.e. brown dwarfs and extrasolar
planets. The last decade has witnessed a revolution in astronomy with the
discovery of these long-sought objects. Observations of dusty disks
around nearby young stars have helped to illuminate how planets form.
Infrared studies of brown dwarfs, objects too low in mass to steadily
produce their own energy, have revealed the properties of very
low-temperature objects, found both free-floating in the solar
neighborhood and as companions to other nearby stars. Finally, direct
detection and characterization of extrasolar planets is becoming possible
through the use of high-contrast adaptive optics imaging on the largest
ground-based telescopes.

Jeffrey
Kuhn is using new techniques for extending the dynamic range
of existing Mauna Kea telescopes (like UKIRT) to look for evidence
of dust or planetary systems around nearby bright stars.

Rolf Kudritzki and Fabio Bresolin study hot massive stars. Such stars,
which are so luminous that they are easily detected in distantg alaxies, have
enormous stellar winds which provide the surrounding interstellar medium with
mechanical energy and momentum and recycledn uclear burned material. Theory predicts
a tight relationship between the momentum rate of the wind and the luminosity
of the star,called the Wind momentum - Luminosity Relation (WLR). A second
relationship, the Flux-weighted gravity - Luminosity Relationship(FGLR) is also
predicted between the fundamental stellar parameters (gravity and temperature)
and luminosity.

Observations and diagnostics of the winds from hot stars, performed
in
our own Milky Way and Local Group galaxies, have confirmed these theoretical
predictions. Drs Kudritzki and Bresolin, together with their colleagues, are
presently carrying out a vigorous observing project (which includes observations
of additional distance indicators such as Cepheids, RR Lyrae stars, the Tip
of the Red Giant Branch and planetary nebulae) to calibrate the WLR and FGLR
as function of spectral type and metallicity. After the calibration phase
is finished, these relationships can be used as new and independent primary
distance indicators, allowing for the measurement of extragalactic distances
out to the Virgo and Fornax galaxy clusters, thus helping to further constrain
the Hubble constant. Hot
stars are also a good way of studying chemical compositions in
distant galaxies (H, He, Mg, Si, C, N, O, Fe, Ti, Cr, Ba etc.),
providing a unique way of understanding the evolution of spirals and
the first step in this direction beyond our Milky Way.

Extragalactic HII regions

Fabio
Bresolin investigates the massive stellar populations
embedded in giant extragalactic HII regions with optical and
near-IR
spectroscopy. The direct spectral signatures of massive objects,
such
as the Wolf-Rayet stars, together with emission-line diagnostics
from
the ionized gas, help us to contrain the stellar mass function,
especially in high-metallicity environments. Information on the
nebular abundances of elements like oxygen, sulphur and nitrogen
in
spiral and dwarf irregular galaxies can be derived and compared
with
abundance determinations from blue supergiants. Such data are a
necessary observational ingredient for the modeling of galactic
chemical evolution.

Planetary Nebulae

Roberto
Mendez and Rolf
Kudritzki study planetary nebulae in our Galaxy and in other galaxies. Planetary nebulae (PNs)
are a brief phase (a few tens of thousands of years) in the late evolution of stars with initial masses
below about 8 or 10 solar masses, immediately before they run out of nuclear fuel and become white
dwarfs. Our Sun will probably produce a PN, but we will have to wait some 5 billion years to witness this.

The PN evolutionary phase is characterized by severe mass loss; the dying star has a dense core (which is about to become
a white dwarf) and a low-density envelope around the core. The envelope is progressively lost in
the
surrounding space. At the beginning of this process we call such
stars "asymptotic giant branch stars". They
are red giants of enormous size and low surface temperature. As
the envelope is lost, the surface temperature of the remaining
core increases until the core becomes so hot that
it emits most of its radiation
in the far ultraviolet. This UV radiation is able to ionize the
hydrogen
atoms in the ejected envelope, which shines in visible light by
fluorescence.
Thus the envelope, which now we call a PN, becomes much brighter,
in visible
light, than its central star. The name "PN" is used for
historical reasons.
The ejected envelope does not form planets; on the contrary, it
may cause
substantial damage to any preexisting planets around the former
red giant
star.

PNs can be counted among the most easily detectable individual
objects
in any galaxy. They have emission-line spectra, easy to recognize
even
if the PN is very faint. Mendez and Kudritzki search for PNs in
elliptical galaxies at distances up to 30 Mpc from us. These PNs
are
very useful because they can provide an accurate measurement of
the distance
to the galaxy where they are found; and their radial velocities
can be used
to study the angular momentum distribution and to test for the
existence and
distribution of dark matter in their galaxies. In the case of the
nearest
elliptical galaxies (closer than 15 Mpc) it may be possible also
to get
some information about the chemical abundances of the PNs, which
may
provide important clues to test current ideas about elliptical
galaxy
formation.

Element Abundances in stars

Ann
Boesgaardand her graduate students use a high-resolution
spectrographs on the Keck telescope to measure the
concentrations
of particular elements in the atmospheres of stars. The three
elements lithium, beryllium and boron are particularly interesting
in that
they
are
formed
by
the action
of cosmic rays in the interstellar medium rather than by nuclear
reactions in the centers of stars. Since these elements
are destroyed by nuclear reactions inside stars at the relatively
cool temperatures of a few
million Kelvins, their abundances in the atmospheres of
stars can provide us with valuable information about
what is going on far below the stellar surface.
For example, Dr Boesgaard has found evidence for rotationally-induced
mixing -- motions deep inside the star that are presumably
caused
by shear flow instabilities below the convection zone -- in stars
warmer than the Sun.

Another useful element is oxygen, whose
abundance in stars of different masses and ages can tell
us much about the history of our Galaxy. Boesgaard's studies
with the Keck telescope have found oxygen enhancements in stars
high above the galactic plane, a result
which indicates that there must have been many high-mass stars
formed when the Galaxy was young.

Other clues to the chemical
history of the Galaxy are to be found the
faint, unevolved stars in globular clusters. These faint stars
reveal the original composition of the gas that they were made
of billions of years ago. Among the elements that have been studied
are lithium, sodium, iron, nickel, chromium, magnesium, calcium,
silicon, titanium, yttrium and barium. Of these, only lithium shows star-to-star
variations: factors of 4 - a challenge for the Big Bang
nucleosynthesis theories

Infrared Stellar Classification

Stars are usually classified by the types of absorption
lines found in their visible spectra; the relative strength of
different absorption lines provide clues to a star's temperature,
surface gravity, luminosity and mass. Some stars, however, are
hidden in dust clouds so thick that they can be seen only by the
infrared radiation they emit. John
Rayner is methodically using the IRTF
to build a reference collection of infrared spectra of stars that
can be used to identify
them when they are obscured. They have observed several hundred
stars of all spectral types between 0.8 and 2.4 microns, and a
smaller number of them out to 5 microns. Some of these reference
stars are also being observed at ultraviolet wavelengths using
the Hubble Space Telescope. The combined spectra make a unique
tool for modelling composite stellar systems and for testing and
improving atmosphere models over a huge wavelength range at all
relevant metallicities, temperatures and luminosities.

Rayner and Cushing are also collecting and studying infrared
spectra of faint stars which are not obscured, but which have temperatures
below about 2000 K. Such objects, which are so cool that almost
of their power is emitted at infrared wavelengths, include the
Brown Dwarfs -- objects which have too little mass to support nuclear
fusion in their cores, but which are too big to be a planet.

ßStars
are so far apart that they do not collide often. The rare collisions
that do occur may result in some exotic astronomical objects. Joshua
Barnes has been throwing virtual stars at each other using
a computer and examining the results in the form of spectacular
movies.